1. Field of the Invention
The present invention relates to a method for producing sinusoidal waves as well as to a circuit device for implementing the method.
2. Description of the Prior Art
In so-called sine wave generators, the need for a low non-linear harmonic-distortion factor usually exists, i.e., the signal power of all the output harmonics together shall be small with respect to the signal power of the fundamental output wave. Sine wave generators with a low distortion factor are required in the measuring of the linearity of a transmission system and for producing a carrier for the data transmission via telephone wires and other supply lines. Furthermore, sine waves with a low distortion factor simplify the control of electrical motors. Basically sine waves may be produced using analog and digital techniques. In analog sine wave generators, an oscillator defines the signal frequency as well as the signal behavior. In order to reduce the distortion factor of the sine wave generator the amplifiers used are allowed to have only a small signal distortion. This limits the frequency range to upper frequency values, allows only a small output power and aggravates integration of the circuit on a semiconductor chip. In a digital sine wave generator an oscillator ony defines the signal frequency, whereas the behavior is controlled by digitally stored information. This information alternatively defines within an oscillator period either the signal amplitude at constant pulse width or the pulse width at constant signal amplitude, respectively. For controlling the signal amplitude linear amplifiers are also required, so that problems arise which are similar to the problems of analog sine wave generators.
A pulse width modulation requires a fine time resolution of the sine wave period in order to produce a signal with low distortion. This requires a high oscillator frequency with respect to the desired sine wave frequency. Since the switching time of power transistors is significant, sine wave generators with pulse width modulation may be used only for low frequencies as a result of the limitations of the switching amplifier.
Furthermore, it is known from the technique of static inverters to convert DC voltages into square wave voltages by means of switch-operated transistors and to convert those square wave voltages by means of low pass filters into a sinusoidal output voltage. In order to eliminate essentially the high content of harmonics within the square wave voltage and in order to produce sine waves with a low distortion factor, high demands are placed on the low pass filter.
Furthermore, it is known from U.S. Pat. No. 3,215,860 as well as from U.S. Pat. No. 3,100,851 to superimpose phase-shifted square wave voltages of equal frequencies with a predetermined amplitude rating in such a way that periodic components of said square wave voltage which contribute to the harmonics of the sine wave to be produced are eliminated, whereas the periodic components which contribute to the fundamental wave are summed up. The methods and circuit devices described there are relatively complex because they need means for phase-shifting of the square wave voltages, for recognition of their amplitudes and for superimposing. Accordingly, it would be desirable to provide a method and circuit device for producing sinusoidal waves which would produce low distortion sine waves utilizing a novel method and simple apparatus which overcomes the aforesaid shortcomings of the prior art.